The purpose of this investigation is to establish the clinical efficacy of sustained-release methylphenidate (Ritalin-SR) as a prelude to a larger-scale, programmatic neuroendocrine, pharmacokinetic, and growth study. The aims are (1) to document the clinical response to Ritalin-SR in hyperactive males, and (2) to establish a dosage equivalency between short-acting and sustained release Ritalin preparations, and (3) to study the attentional, cognitive, and behavioral responses to the two preparations using a within-subjects crossover design. It is hypothesized that a single Ritalin-SR daily dose will be exactly equal, milligram for milligram, to the total of two divided doses of the short-acting formulation, given four hours apart. A sample of 50 Ritalin-responsive, hyperactive males with ages ranging from 8 to 12 years will be recruited as phase one of a double-blind, crossover study. Following a two week placebo washout period, the study's second phase will begin as the sample undergoes semi-random assignment (pairs matched by age), with half the sample placed back on their standard dose of short-acting Ritalin, and the remainder treated with Ritalin-SR. The second group's dosages will be adjusted on the basis of teacher's reports for optimal clinical benefit, while the short-acting Ritalin group's dose will be fixed. After a second two-week placebo washout, the two groups will be crossed over to phase three, a two-week, fixed dose study for comparison of both drugs at optimal and equivalent mg/kg dosages. Dependent variables will include global teacher's ratings, direct behavioral classroom observations, laboratory attentional measures, and paired associates learning tasks for each drug study phase. Data analysis will utilize analysis of variance to compare within-group and within subjects behavioral, cognitive, and attentional responses.